The world continues to rely heavily on energy supplied by fossil fuels such as petroleum, coal and natural gas, and the forecast for future energy supplies does not indicate that the use of these three sources will diminish substantially in the coming years. The increasing use of these energy sources releases huge amounts of carbon dioxide in the atmosphere every year, and there is increased pressure throughout the world to limit these releases as a consequence of the real or perceived impact of CO2 on global climate change.
A primary method to limit the release of CO2 is to trap it at its release point for possible storage via one of several potential sequestration technologies. A key roadblock is the development of cost-effective CO2 capture/separation technologies, as these components of the process are expected to represent 75% of the total cost in oceanic or geologic sequestration methodologies. Excellent CO2 separation technologies have been developed for acid gas removal from underground natural gas sources. This classic method uses aqueous amines to absorb CO2, and a similar technology has been suggested for CO2 capture from power plant effluents. However, the amine regeneration step, whereby the CO2 is removed from the amine solution, is extremely energy intensive, making the cost of using this technology for CO2 capture from flue gas streams too high. An alternate approach that is potentially much less costly is to apply solid adsorbents, for example in a fixed bed, for the adsorption of CO2 from these streams. Indeed, many different solid adsorbents have been studied, including zeolites and strongly basic oxides such as hydrotalcites and MgO. However, the performance of these solids has not been satisfactory owing to problems arising from preferential water adsorption (on zeolites) or the high temperatures needed (for basic oxides).
An alternative technology that has only just begun to receive attention is the use of amine-functionalized silica or polymeric materials. These solids are inexpensive and operate well in the presence of water vapor, a requirement for separation of CO2 from combustion streams. Most amine-functionalized silica materials have relatively limited capacities for CO2 capture due to the small number of amines incorporated into the adsorbent material. The amine site density is normally limited by the surface area of the solid, which regulates the number of aminosilane molecules that can be used to graft the amine sites onto the solid support. Typical amine loading for these types of sorbents are 0.3-3 mmol/g solid. Thus, low amine loadings are a key factor limiting CO2 capacity. Higher capacities can be achieved by physisorbing/impregnating poly(ethyleneimine) or other polymeric amines on silica or other supports. However, in this case, a lack of a covalent linkage to the support makes the long term stability of the material problematic.
Therefore, there is a need in the industry to overcome at least some of the aforementioned inadequacies and deficiencies.